The invention relates to a programmable hearing aid with an amplifier section and a transmission section whose transmission characteristics can be controlled. This is the state of the art.
Thus, for instance, from U.S. Pat. No. 4,187,413, corresponding to DE-OS No. 27 16 336, we are familiar with a hearing aid (HDO unit) on which the analog sound signal routed from the microphone, after passing through a low-pass filter, is converted to a digital signal in an analog-to-digital converter and routed to a discrete signal processing circuit whose transmission function of the nth order can be controlled on the basis of parameters stored in an erasable, programmable read-only memory (EPROM) by means of a microprocessor with an arithmetic unit, for matching to the auditory damage. The programming can be changed only by erasing the contents of the EPROM and reprogramming it. The digital signal, thus modified, is then converted back to a corresponding analog signal in a digital-to-analog converter, amplified and routed to the earphone. This unit can be matched only to an individual, pre-determined and quite specific auditory damage level at any one time, determined beforehand with an audiometer. In addition, from EU No. 0064 042 A 1, we are familiar with a circuit arrangement, preferably for a hearing aid, in which the parameters for 8 different ambient situations, for instance, are stored in a memory. The first group of parameters is recalled, for instance, by operating a switch and controls a signal processor connected between the microphone and earphone via a control unit, and the signal processor then sets the first transmission function determined for a scheduled ambient noise situation. All 8 transmission functions can be recalled consecutively via a switch until the correct transmission function is found. On the other hand, an automatic matching facility is provided if the user, for instance, leaves a noisy environment and enters a quiet environment or vice versa.
If one wishes to set other transmission functions, the contents of the non-volatile memory must be erased and the memory must then be reprogrammed with an external programming unit. In this case also, it is possible to match the unit to a changing auditory damage level only with great difficulty.
Further, more detailed tests aimed at providing an improved transmission quality for hearing aids by using a specially developed voice processor with voice compression and separation of voiced and unvoiced sounds are described for instance in IEEE Transactions on Acoustics, Speech and Signal Processing, December 1976, Pages 473 to 480 in the paper by S. G. Knorr "A Hearing Aid for Subjects with extreme High-Frequency Losses".
One further test aimed at improved matching of hearing aids is described in a paper by A. Rihs and D. Wessell "Active Filtering - a step towards the programmable hearing aid" in "Hearing Instruments" Volume 33, No. 10, 1982, Pages 20 and 56.
With the filter arrangement described in the above paper, with a filter stage connected between the microphone plus pre-amplifier and output stage with earphone, utilizing the principle of feedback with phase rotation, the filter stage, in the feedback path between output and input, uses a filter network which is controlled by a programming switch which, in turn, controls groups of various circuits. Each setting of the programming switch can be locked in position (detent) and calibrated to a close tolerance. This also permits matching to the auditory damage level of the patient which is admittedly better but, which can be carried only once.
A further filter system for hearing aids is discussed in detail in US Pat. No. 4,188,667. With this filter system, the pass-band characteristic can be matched to an arbitrarily selected spectrum. In one embodiment, the filter system is located outside the hearing aid and is connected to it by two high-frequency transmission channels, each with transmitter and receiver. The external filter system can also be connected via wires instead of the wireless connection.
All these known arrangements are designed either only for a single transmission function or for a pre-determined number and type of transmission functions, and some of them are still equipped with extremely expensive and complex filter systems and, in certain cases, also require relatively large read-only memories whose contents can be erased and reprogrammed.